July
25, 2003 ó A team of scientists from NOAA,
the Louisiana Universities Marine Consortium (LUMCON),
and Louisiana State University
is forecasting that the size of the “Dead Zone” off the
coast of Louisiana and Texas this summer should be between 4,770 and
6,900 square miles, an area approximately the size of the state of Connecticut.
(Click NOAA image for larger view of “dead zone”
in the Gulf of Mexico in July 2002. Please credit “NOAA.”)

The “Dead
Zone” is the name for the seasonal change in areas of the Gulf
of Mexico where algal growth, stimulated by input of nutrients such
as nitrogen and phosphates from the Mississippi and Atchafalaya rivers,
settles and decays in the bottom waters, leading to decreased oxygen
levels. Because the decaying algae consumes oxygen faster than it can
be replenished from the surface, the result is oxygen levels too low
(hypoxia) to support most life in a massive area two times larger than
the entire surface of the Chesapeake Bay.

The prediction
for this summer is an area slightly larger than the average annual hypoxia
affected waters since 1990 of approximately 4,900 square miles. The
forecast is based on nutrient loads from the Mississippi and Atchafalaya
Rivers in May and June provided by the U.S. Geological Survey.

This
is the first advance forecast of the annual hypoxic event in the Gulf
of Mexico and is an example of an innovative environmental service—officially
referred to as “ecological forecasting”—that NOAA
scientists believe will become an important tool in coming years for
both decision makers and the public. (Click NOAA image for larger
view of “dead zone” in the Gulf of Mexico from June 11 through
July 17, 2003. Please credit “NOAA.”)

Oceanographers
have been exploring new ways to provide more accurate and timely forecasts
of the “Dead Zone,” and their research has led to the development
of a forecasting model that was published in the May 2003 issue of the
peer-reviewed journal Limnology and Oceanography. The model is the first
to predict directly the size of the hypoxic zone as a function of changes
in nutrient loadings.

“By
using this river dissolved oxygen model based on Mississippi River nutrient
loadings in the northern Gulf of Mexico, our research team was also
able to look back more than 30 years and determine that these now virtually
perennial events were uncommon before the mid-1970s,” said lead
author Donald Scavia, the chief scientist of the NOAA
Ocean Service.

The northern
Gulf of Mexico’s bottom-water summer hypoxic zone in recent years
has extended roughly 375 miles westward from the mouth of the Mississippi
River, in Louisiana, across the Texas border.

Between
1985 and 1992, the northern Gulf of Mexico hypoxic zone averaged 3,200
square miles (8,300 km2). But between 1993 and 2001 that hypoxic region
nearly doubled—to an annual average of roughly 6,200 square miles
(16,000 km2), according to research done by LSU scientist Nancy Rabalais,
one of the report’s co-authors.

Research
cruises to track development of hypoxia have been conducted monthly
since January. Rabalais, chief scientist for hypoxia research at LUMCON,
explains that the “algal
blooms that fuel the eventual summer hypoxia events were abundant
this spring when the Mississippi River discharge peaked. Another crest
of the river, in late May, provided more nutrients for another boost
of algal growth. Hypoxia began to form in April and May and was widespread
in early June.” Rabalais notes, however, that two recent tropical
storms have caused a shrinking of the hypoxic area where the storm passed
across the southeastern Louisiana shelf but that hypoxia was once again
rebuilding. This year’s research cruise to map the extent of the
hypoxic, or low oxygen, zone is currently underway.

The NOAA funded research team says a nearly tripling in nitrogen loadings
to the Gulf over the past 50 years has led to the heightened Gulf of
Mexico hypoxia problem. The scientists say their research will make
possible improved assessments of hypoxia effects under various Gulf
Coast oceanographic conditions. The study suggests that a 30 percent
reduction in nutrient loadings over a five-year running average, as
currently suggested in the 2001 Nutrient Task Force report, would lead
to a 20 to 60 percent reduction in the Gulf Coast area experiencing
the “Dead Zone” phenomenon.

This work
evaluated the influence of nitrogen load and variations in ocean currents
looking both backward and forward at the range of the Gulf’s hypoxic
area relative to changing nitrogen loadings.

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